Method for measuring roll crown and improving the operation of a rolling mill



METHOD FOR MEASURING W. SMITH, JR

ROLL CROWN AND IMPROVING THE OPERATION OF A ROLLING MILL Filed Sept. 28,1965 Andrew W. Smifh,Jr.

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X -ROLL GAP BELOW FACE (INCHES') A BY (RF/ ATTORNEY United States Patent3,387,470 lillil'llHOD FGR MEASURING BULL CROWN AND IWgPRGWNG THEOPERATION OF A RULLING M LL Andrew W. Smith, In, Pittsburgh, Pa.,assignor to Westinghouse Electric Corporation, Pittsburgh, lPa., acorporation of Pennsylvania llliied Sept. 28, 1965, Ser. No. 4%,832 11Claims. (Cl. 72-8) ABSTRACT OF THE DISCLUSURE The roll crown of workrolls at a steel mill rolling stand is determined from time to timeduring stand use. Use is made of the roll crown in setting screwdownposition for desired strip crown and desired delivery gauge.

The present invention relates to reduction rolling mills and moreparticularly to methods for measuring roll crown so as to provideimprovement in controlling the shape of rolled strip products.

Roll crown is a mill operating parameter which is one of numerousdeterminants of the shape of rolled product from the mill. Normally, thework rolls are originally formed with a crown in that the diameter ofeach work roll is enlarged at the center of the roll and decreasestoward each end of the roll to smaller roll end diameters.

Generally, roll crown provides shape control in offsetting the effectsof roll bending produced by strip transported between the rolls. Inturn, the extent of roll bend-- ing is determined by the magnitude ofthe applied roll force which depends on the roll opening, the propertiesand characteristics of the material being rolled, and other parameters.The strip gauge reduction or draft produced by the rolls is alsodetermined by the applied roll force and the properties andcharacteristics of the material being rolled and certain otherparameters, and desired strip gauge and shape are both realized onlywhen the variables including roll crown are suitably accounted for orcontrolled.

Percent strip crown is defined as the extent to which the stripthickness along its center line exceeds the strip thickness along itsedge lines divided by the average strip thickness times 100. Desirably,the strip cross-section shape is controlled to have about 1% crown sothat the strip can be guided stably between the work rolls substantiallywithout side movement. In a multistand mill, the percent strip crown ispreferably held substantially constant at least throughout the last fewpasses so that differential elongation and buckling or non-flatness areavoided in the finished product.

The roll crown varies as a function of roll temperature changes andwear, and strip shape accordingly also varies at desired gauge unlesssuitable correction is made. The wear of back-up rolls produces agenerally equivalent effect on strip shape since the geometry of theback-up rolls affects the extent to which the work rolls are bent undera given separting force.

In US. Patent 3,248,916 Work Piece Shape Control with a Rolling Mill andissued on May 3, 1966 to A. F. Kenyon and the present inventor andassigned to the present assignee, a computer control system is arrangedto operate a rolling mill such that strip product is rolled withimproved gauge and shape control. Successive gauge reductions or draftsat successive stands are set by the computer within constraint limits ofroll force, drive motor torque, maximum percentage draft and maximuminches draft while strip crown is controlled such that substantiallyfiat on gauge product is produced.

In the referenced computer control system, roll crown lifidld'idPatented .lune fl, 1968 ICC variance is accounted for in the computerprogram by the insertion of an equation correction factor estimatedafter visual observation of any tendency for non-flatness to occur inthe rolled product. In other less automated control systems, the milloperator may normally rely on his accumulated experience in determiningwhen and to what extent mill settings must be altered to offset thechanging roll crown variable during mill use. For example, in a hotstrip steel mill it is generally known that, after the first hour or soof use of a new set of work rolls, the roll crown increases due toinitial differential heating between the center and edges of the workrolls. Thereafter, the roll crown decreases from wear until the roll setmust be rep-laced with a new roll set, perhaps after eight to sixteenhours of use. As previously noted, the back-up rolls also play a part indetermining the effectiveness of the work roll crown, and the wearhistory of the backup rolls in use may be considered by the operator injudging the settings to be made as the rolling mill continues inoperation.

In both the computer and the operator control systems, I

only the original or starting work roll crown is known and subsequentmill settings are determined from estimates of the roll crown. Althoughthe experiential factor in compensating for variable work roll crown canprove to be an acceptable mode for mill operation, it is generallyundesirable since commercial and economic factors demand a morescientific approach.

In accordance with the broad principles of the present invention, animproved rolling mill operating method is provided and includes a uniquemethod for measuring work roll crown during the use of the work rolls.The operating method further includes using the in process crownmeasurement in a computer or other mill control system substantially toprovide desired strip guage and shape.

it is therefore an object of the invention to provide a novel method foroperating a rolling mill so as to produce strip product with improvedshape control.

Another object of the invention is to provide a novel method foroperating a rolling mill wherein work roll crown is measuredperiodically for use in controlling the strip shape.

A further object of the invention is to provide a novel method formeasuring roll crown in a rolling mill so that the actual roll crown canbe made known for control use during the mill operation.

These and other objects of the invention will become more apparent uponconsideration of the following detailed description along with theattached drawings, in which:

FIG. 1 is a schematic diagram of a multistand rolling mill which can becontrolled in accordance with the principles of the invention;

FIG. 2 shows an elevational view of a set of work rolls and a set ofback-up rolls employed at the various stand locations in the mill shownin FIG. 1; and

FIG. 3 shows an exemplarly plot of roll force versus roll gap, and it isuseful in the measurement of work roll crown during the operation of arolling mill in accordance with the principles of the invention.

More specifically, there is shown in FIGURE 1 a multistand reductionrolling mill comprising a plurality of roll stands St and S2 through S6.At each stand location, a pair of work rolls 12 and a pair of back-uprolls 14 are provided for rolling in this instance hot steel strip 16.The rolling mill 10 can be arranged to roll other plastically deformablematerials and other forms of material such as plates, slabs, etc.Suitable motor drives 17, such as variable speed DC motors, operate therolls 12 and 14 at the respective stand locations under suitable controlby a conventional speed control system (not specifically shown).

A conventional screwdown position control 18 is employed at each standlocation so as to control the force applied to the back-up rolls 14. andthereby control the gauge and shape of the strip 16 as it passes throughthe opening between the work rolls 1?. as is well known in the art.Generally, the roll force is controlled in accordance with the wellknown roll force principle so as to maintain a substantially constantpredetermined roll opening through control of the screwdown position.Strip shape is generally controlled by establishing a roll opening whichresults in the roll bending required for desired strip crown at aparticular roll crown value. A conventional tension control system (notshown) can also be provided if desired so as to regulate the motor drivespeeds to retain strip tension in a desired range.

Each of the screwdown position controls 18 is in this instancecontrolled by a central digital process computer 20 in conjunction withsuitable analog-to-digital and digital-to-analog conversion circuitry.Screw position and roll force signals as indicated by the referencecharacters 30 and 32 are transmitted to the computer 20 from eachscrewdown position control 18 for use with a gauge monitor signal from adelivery thickness or X-ray gauge 22 in computing corrective screwdownposition changes. The computer 20 is suitably designed and programmed toachieve the control required to produce substantially on gauge product.It is also suitably arranged generally to control the overall operationof the mill including the mill drive speed through control of the drivespeed control subsystem as indicated by the reference character 24. Thepreviously mentioned tension control subsystem and other providedsubsystems are also amenable to computer control if desired.

In addition to the gauge control signals, various other signals relatedto on-going process variables are connected to the computer input so asto achieve the desired output controls. For example, speed signalsindicated by the reference character 26 are so connected. The computeralso provides output control in response to a data input device such asa commercially available tape reader 38 or a suitable manual inputcontrol 40 which generates suitable data representative of thecharacteristics of the particular strip being rolled. Operatingrestraints similar to those previously indicated can be programmed inthe computer 20 if desired. Initial data for the overall computerprogram can include workpiece entry thickness and length, temperature,width (which directly affects roll bending) and type of material.

Since work roll crown is a determinant of rolled strip shape, it isincluded as a factor in the program of the computer 20. Flat rolledproduct can thus be obtained at substantially desired gauge and withsubstantially desired shape. To the extent that the crown of the workrolls 12 changes at any particular stand location, the screwdownposition control 18 at that stand can be operated by the computer 20 (asindicated by the reference character 28) so as to adjust the screwdownposition and the work roll setting in the interest of maintainingdesired strip crown.

An enlarged view is shown in FIGURE 2 so as to illustrate the originalcrown provided for the work rolls 12. Thus, each work roll 12 isprovided with a center diameter D and with respective end diameters DRoll force is applied to the work rolls 12 by means of the back-up rolls14. Half of the total roll force is applied at each end of each back-uproll 14, and the total roll force is applied to the work rolls 12 overthe intersurface contact area between the work rolls 12 and the back-uprolls 14. As indicated previously, the original work roll crown isordinarily precisely known since it is accurately formed on the workrolls 12 before they are placed in service.

In order to measure the work roll crown during mill service, arelationship between total roll force and roll gap below face isemployed. By roll gap below face, it is meant to refer to screw positionmovement following first contact between the work rolls 12.

The roll force and roll gap data are taken in the absence of strip orother forms of rolled material between the work rolls 12. Thus, the rollforce and roll gap data form an empty mill spring curve as exemplaryshown in FIG- URE 3 for a 134 inch plate mill.

In the mill operating and roll crown measuring methods of the invention,new work rolls 12 are submitted to a roll crown test as indicated by themill spring curve 4-2. Roll force and roll gap below face are determinedby suitable signals such as the signals 30 and 32. Work roll contact isfirst established as indicated by the reference character 4 2, and totalroll force F increases nonlinearly as the roll ap below face increasesuntil play in the screw threads and other support structure is taken up.The relationship between total roll force and roll gap below face thenbecomes linear as indicated by the reference character 46. In theinitial test, roll force is increased until an arbitrary force value isrealized, for example 2 million pounds as indicated by the referencecharacter 48. The roll gap below face is noted at that point to be .10inch.

The linear portion of the line, if extended, intersects the X axis atreference character 57. The distance X57 is determined by several fixedconditions such as play in the screw threads and other support structureand the variable work roll and backup roll crown. The detection of thevariation in the nonlinear portion of this line is in essence the basisof determining variation in roll crown.

After the work rolls 12 are placed in use, the empty mill spring curvecan be periodically redetermined so as to provide an indication of achange in the roll crown. The test can be quickly made between plate orstrip or other workpiece passes. For example, after two hours of use orwhen it is otherwise expected that the work roll crown will haveincreased as a result of differential roll heating, a new test is run todetermine a new empty mill spring curve 50, and when the roll forcereaches the preselected figure of 2 million pounds, as indicated by thereference character 52, the roll gap below face X is noted to be .11inch. The change in roll gap below face is given by the following:

In turn, the change in roll crown is given by the following formula:

It is ordinarily safely assumed that the total change in roll crownmeasured by the described procedure is approximately equally dividedbetween the two work rolls 12.

At a later time in the mill operation when it is expected that the workroll crown will have decreased due to roll wear, another test can bemade to produce an empty mill spring curve indicated by the referencecharacter 54. When the roll force reaches the preselected value of a 2million pounds as indicated by the reference character 56, the roll gapbelow the face X is again noted and in this case it is .09 inches. Thechange in roll crown is in this case inch. The minus sign indicates adecrease in roll crown from the original value.

The roll crown measurement test can be performed as often as desiredduring the operation of the rolling mill. It can be performed for thesingle set of rolls in a reversing mill or for preselected sets of rollsor each set of rolls in a multistand rolling mill. The roll crownmeasurement test can be manually instituted and the results can beemployed by the operator of a non-computerized mill for the purpose ofadjusting the screwdown position settings to maintain desired stripcrown while work roll crown varies during mill operation. Further, thetest results can be used to operate a suitably designed analog feedbackcontrol (not shown) especially arranged to produce the same compensatorycontrol of the screwdown settings.

In a computerized mill, such as the mill of FIG. 1, the computer 20 canbe programmed to institute roll crown tests at the various roll standsand at various times during the mill operation. During the roll crowntests, the screw position setting signals 30 and the roll force signals32 generate the necessary mill spring curve data for the computermemory. When the roll crown change is calculated for each stand aspreviously indicated, a new roll crown parameter is made available toupdate the computer control equations for the various stands. Thecomputer 20 thereby applies feedback control to the screwdown positioncontrols 18 as indicated by the reference character 28 so as tocompensate for roll crown change in response to the roll crown testresults. For example, the computer 20 can contain a program including inpart the following general equation which is used for each standlocation:

Iz=entry strip thickness in inches;

As=percent strip crown desired;

k =updated roll crown in inches;

F =modulus of elasticity for roll bending, pounds/inch;

F :force required to produce enough roll bending to remove the rollcrown and impart enough additional bending to produce the desired stripcrown.

Updated values of k for each stand location cause changes in commandvalue for roll force at that stand. Screwdown position is thereforevaried to provide the command roll force and the programmed compensationfor changes in work roll crown. From an overall stand point, thecomputer 20 prescribes a general draft pattern for the successive millstands in accordance with the workpiece input characteristics, andcontrols the draft or gauge reduction taken at each stand in the mill sothat substantially desired delivery gauge is realized while strip shapeis adequately maintained at the various stand locations to result inflat oil-gauge product. Although roll force changes made to compensatefor roll crown changes and to control strip shape from a particularstand have an effect on strip gauge from that stand, the computcr 20interrelates the functioning of the various stands so as substantiallyto provide the desired finished product.

When the roll crown tests are conducted with empty work rolls, the millspring constant can vary from time to time as a result of wear of theback-up rolls 14. Since back-up rolls characteristically have a life ofone week or more, it can be safely assumed that the slope of the millspring constant is substantially constant for the relatively shortperiod of time during which comparative tests are made for the workrolls.

In summary, an improved method is provided for operating a reductionrolling mill so that better shape control is provided in the finishedproduct. The method includes a unique procedure for determining actualwork roll crown after the work rolls are installed for operation andusing the test results for automatic or other adjustment of the millcontrol system in producing rolled product of substantially desiredgauge and shape.

The foregoing description has been presented only to illustrate theprinciples of the invention. Accordingly, it is desired that theinvention not be limited by the embodiment described, but, rather, thatit be accorded an interpretation consistent with the scope and spirit ofits broad principles.

What is claimed is:

1. A method for operating a reduction rolling mill subiect to gauge andshape control and having at least one reduction stand with at least onepair of work rolls, said method comprising the steps of determining atleast a reference point on the empty mill spring curve for the reductionstand at a predetermined point in the life of the Work rolls,determining at least a point on the empty mill spring curve for thereduction stand at a point in the roll life after the work rolls haveundergone subsequent use, determining any change in work roll crown fromthe test points, and using any determined roll crown change inprescribing the setting of the work rolls.

2. A method for operating a reduction rolling mill subject to gauge andshape control and having at least one reduction stand with at least onepair of work rolls, said method comprising the steps of determining atleast a reference point on the empty mill spring curve for the reductionstand at a predetermined point in the life of the work rolls,determining at least respective points on the empty mill spring curvefor the reduction stand at various points in the roll life after thework rolls have undergone subsequent use, determining any changes inwork roll crown from the test points, and using any determined rollcrown change in prescribing the setting of the work rolls.

3. A method for operating a reduction rolling mill subject to gauge andshape control and having at least one reduction stand with at least onepair of work rolls, said method comprising the steps of determining atleast a reference point on the empty mill spring curve for the reductionstand prior to any significant rolling use of the work rolls,determining at least a point on the empty mill spring curve for thereduction stand at a point in the roll life after the work rolls haveundergone subsequent use, determining any change in the work roll crownfrom the test points, and using any determined roll crown change inprescribing the setting of the work rolls.

4. A method for operating a reduction rolling mill subiect to gauge andshape control and having at least one reduction stand with at least onepair of work rolls, said method comprising the steps of determining atleast a reference point on the empty mill spring curve for the reduction stand prior to any significant use of the work rolls,determining at least a point on the empty mill spring curve for thereduction stand at a point in the roll life after the work rolls haveundergone subsequent use and have expectedly acquired an increased crowndue to differential roll heating, determining at least a point on theempty mill spring curve for the reduction stand at a point in the rolllife after the rolls have undergone additional subsequent use and haveacquired an expectedly decreased crown due to roll wear, determining anychange in work roll crown from the test points, and using any determinedroll crown change in prescribing the setting of the work rolls.

5. A method for operating a reduction rolling mill subject to gauge andshape control and having at least one reduction stand with at least onepair of work rolls, said method comprising the steps of determining atleast a reference point on the empty mill spring curve for the reductionstand at a predetermined point; in the life of the work rolls,determining at least a point on the empty mill spring curve for thereduction stand at a predetermined point in the roll life after the workrolls have undergone subsequent use, determining any change in work rollcrown from the test points, and operating an automatic gauge and shapefeedback control system in response to any roll crown change indetermining the setting of the work rolls.

6. A method for operating a reduction rolling mill subject to gauge andshape control and having at least one reduction stand with at least onepair of work rolls, said method comprising the steps of determining atleast a reference point on the empty mill spring curve for the reductionstand at a predetermined point in the life of the work rolls,determining at least respective points on the empty mill spring curvefor the reduction stand at various points in the roll life after theWork rolls have undergone subsequent use, determining any changes inwork roll crown from the test points, and operating an automatic gaugeand shape feedback control system in response to any roll crown changein determining the setting of the work rolls.

7. A method for operating a reduction rolling mill subject to gauge andshape control and having at least one reduction stand with at least onepair of work rolls, said method comprising the steps of determining theroll gap below face produced by a predetermined roll force at apredetermined point in the life of the work rolls, determining the rollgap below face produced by the same predetermined roll force at a pointin the roll life after the work rolls have undergone subsequent use,determining any change in work roll crown from any change in the rollgap below face measurements, and using any determined roll crown changein prescribing the setting of the work rolls.

8. A method for operating a tandem reduction rolling mill subject togauge and shape control and having a plurality of tandem reductionstands each with at least one pair of work rolls, said method comprisingthe steps of determining at least a reference point on the empty millspring curve for at least preselected ones of the reduction stands at apredetermined point in the life of the Work rolls associated with thepreselected stands, determining at least a point on the empty mill curvefor the same reduction stands at a point in the respective roll livesafter the respective work rolls have undergone subsequent use,determining any change in the roll crown of the respective work rollsfrom the respective sets of test points, and using any determined rollcrown change data in prescribing the setting of the work rolls.

9. A method for operating a reduction rolling mill as set forth in claim8 wherein the method further includes the steps of determining at leasta point on the mill spring curve for the preselected reduction stands ata plurality of points in the lives of the respective sets of work rollsassociated with the preselected stands and after the respective sets ofwork rolls have undergone subsequent use, and operating programming aprogrammed computer included in an automatic gauge and shape feedbackcontrol system for the mill so as to operate a control system inprescribing the setting of the work rolls in response to any determinedroll crown change.

It A method for determining the change in roll crown after a set ofreduction mill work rolls have undergone a period of use, said methodcomprising the steps of determining at least a reference point on theempty mill spring curve prior to any significant use of the work rolls,determining at least a point on the empty mill spring curve at a pointin the roll life after the work rolls have undergone subsequent use, anddetermining any change in work roll crown from the test points.

11. A method for determining the change in roll crown after a set ofreduction mill work rolls have undergone a period of use, said methodcomprising the steps of determining the roll gap below face produced bya predetermined roll force prior to any significant rolling use of thework rolls, determining the roll gap below face produced by the samepredetermined roll force at a point in the roll life after the workrolls have undergone subsequent use, and determining any change in workroll crown by multiplying any difference in the roll gap below facemeasurement by one half.

References Cited UNITED STATES PATENTS 11/1966 Perrault et a1. 72-87/1967 Beadle et al 727

